It is known as a function of ultrasonic diagnostic apparatuses that ultrasonic pulses are transmitted from a surface of a skin of a subject toward a blood vessel of the subject, so as to carry out a measurement on various states of the blood vessel based on ultrasonic echo signals reflected by the blood vessel.
For example, a method of detecting the boundary of a blood vessel wall by ultrasonic waves is disclosed in JP 2000-271117 A. According to this method, assuming that a blood vessel has a standard structure, the displacement and diameter of a blood vessel, the thickness of a blood vessel wall, and the like are measured based on a maximum peak value and a second peak value of a luminance signal in image data obtained based on ultrasonic waves reflected by the blood vessel.
Further, a method of calculating a propagation speed of pulse waves in a blood vessel is disclosed in JP 11 (1999)-76233 A. According to this method, a propagation speed of pulse waves is determined based on a variation with time in the cross-sectional shape of a blood vessel in a plurality of regions in a longitudinal direction of the blood vessel.
Further, Japanese Patent No. 2889568 discloses a method of detecting an IMT (Intima-Media Thickness) value of a blood vessel wall (carotid artery) by ultrasonic waves. According to this method, assuming that a blood vessel has a standard structure, an IMT value of a blood vessel wall (carotid artery) is measured based on a maximum peak value and a second peak value of a luminance signal in image data obtained based on ultrasonic waves reflected by the blood vessel.
However, in an actual medical diagnosis of a blood vessel, it is difficult in many cases to carry out a measurement in a stable state due to a variety of factors such as a state in which an ultrasonic probe is fixed, a stationary and respiratory state of a subject, and the like.
On this account, in an actual measurement, a large-scale device is used to fix a probe and a subject and the subject is requested to stop his/her breathing during the measurement, so that a measurement is more likely to be carried out stably. Further, the measurement is carried out a plurality of times and data that can be measured with accuracy are selected. Consequently, the measurement takes a long diagnosis time, and a constant measurement accuracy cannot be obtained by different operators. Also, for future use in the area of a diagnosis targeted for a large number of people, such as a medical examination, means for judging a state of stability of a measurement is being demanded.